For purposes of explanation, the present description is made in the example of an image forming apparatus such as an electrophotographic apparatus. In a conventional charging step in an electrophotographic process using an electrophotographic photosensitive member, generally, a high voltage (DC voltage of about 5 to 8 kilovolts) is applied to a metal wire to generate a corona, which is used for charging the photosensitive member. In this method, corona discharge products such as ozone and NO.sub.x are generated along with the generation of the corona. These corona discharge products deteriorate the surface of the photosensitive member thereby causing image quality deterioration such as image blur or image fading. Further, because the contamination on metal wire affects the image quality, there has been problems such as white droppings, dropouts or black streaks in the resultant copied image. In addition, the corona wire requires regular maintenance and cleaning.
Moreover, as most of the current flows to a shield plate disposed around the metal corona wire, the proportion of current directed to the photosensitive member is generally 5-30% of the consumed current. As a result, the conventional corona charging method has a low electrical power efficiency.
An alternative to the corona discharge wire is a contact-type charging device. Contact chargers have the advantage of reduced voltage requirements as well as reduced amounts of generated ozone. In contact chargers, a charging member, to which voltage is applied, is brought into contact with the surface of a member to be charged so as to charge the member.
These contact chargers have a multi-layer construction including a resilient layer about a rigid conducting shaft, a resistive layer formed on the resilient layer, and a third outer layer on the resistive layer to provide the necessary contact characteristics. The inner resilient conductive layer includes electrically conductive particles such a carbon black, graphite and metallic powder dispersed in an elastic material, such as rubber or resin to provide an electrical conductivity (resistivity). The resistive layer ensures an appropriate surface electrical resistance and the inner resistance layer retains appropriate elasticity and resistivity (conductivity) to provide an optimal nip width with respect to the surface of the member to be charged.
Sufficient elasticity is necessary to charge the member, such as a photosensitive drum, and to prevent leakage caused by pinholes in and damage to the surface of the photosensitive member. The resistive layer also includes dispersed electrically conductive particles which tend to become unstable and may vary from portion to portion of the layer.
Alternatively, an electrically conductive rubber roller may be formed by adding a silica type filler, carbon black, a foaming agent, a plasticizing additive, a vulcanizing agent and the like to a rubbery siloxane polymer, subjecting it to transfer forming, injection molding, performing, extrusion molding and the like, whereby it is cross-linked and made into roller form. These and similar charging members are disclosed in U.S. Pat. Nos. 5,313,359; 5,241,343; 5,126,913; 5,402,213; 5,443,873; and 5,017,965, herein incorporated by reference.
However, these prior designs are unable to provide repeated or multiple cycles for a given cartridge. That is, the alignment of the carbon or conductive fibers in the conductive elastomeric layer may deteriorate with use, thereby providing a nonuniform charge distribution in this layer. Further, upon the wearing of the outer resistive layer, any residual "hot spots" in the underlying resilient layer are exposed to the photosensitive member. Therefore, the need exists for a contact charger having a more nearly uniform charge distribution throughout the contact surface and any underlying surface. The need further exists for a contact charger which can be readily formed having sufficient tolerances to reduce deterioration of the printing quality through repeated cycles.